In hydraulic brakes of this type there is the problem that the calibrated passage must be so dimensioned that, on the one hand, the rapid braking required for emergency braking is assured, and that, on the other hand, shocks and jolts during the braking process must be limited to levels acceptable for passengers in other vehicles which may coupled to the vehicle being braked.
The problem to be solved is illustrated in FIGS. 1 and 2.
FIG. 1 is related to a hydraulic brake in which the calibrated passage determining pressure increase during emergency braking is relatively large. If, with released brakes at time 0, the emergency brake valve is switched to its emergency braking position, time interval 0-A, referred to as dead time, elapses while the relatively large dead volume of the brake cylinder fills with hydraulic medium and the piston in the brake cylinder performs its idle stroke, until the vehicle brakes are applied when time A is reached. Hence, during the interval 0-A, there is no significant rise in braking pressure. After application of the vehicle brakes, the hydraulic medium continuing to flow into the brake cylinders through the calibrated passage firmly applies the brakes, if appropriate with elastic loading of the hydraulic and mechanical braking elements, causing a rather rapid rise in brake pressure until moment B. At this point, the maximal brake pressure acceptable by the brake cylinders is reached; this may correspond to the pressure of the hydraulic accumulator or the switching pressure of a conventional pressure limiting valve downstream of the accumulator and possibly subject to additional control dependent on vehicle load. From moment B on, the brake pressure remains substantially constant.
As already mentioned, FIG. 1 is based on a relatively large calibrated passage, which assures relatively rapid brake application, so that dead time 0-A is short. The relatively large calibrated passage also assures rapid increase in braking pressure during the clamping phase of the vehicle brake; for this reason, the pressure increase gradient starts very suddenly and rises very steeply. The rapid transition and the steep pressure increase gradient result in a large shock and jolt load during the braking process, which is not acceptable for the passengers of the vehicle. In the case of several coupled vehicles, there is the additional danger that the brakes of the individual vehicles are applied at slightly different moments, causing further shocks or jolts between the coupled vehicles due to abutment or drag phenomena.
In order to prevent large shock or jolt loads during emergency braking, it seems desirable to use a relatively small calibrated passage, so that after brake application, through the avoidance of a rude transition, there is a gradual increase in braking pressure until maximal brake pressure is attained. This is shown schematically in FIG. 2. In this case, there are lesser delay changes for the vehicle, and differential braking developments on coupled vehicles lead to lesser braking differences. In short, there is emergency braking in which the shocks and jolts are limited to acceptable values. It is however clear from FIG. 2 that the narrow calibrated passage so throttles the inflow of hydraulic medium into the brake cylinders that a long dead time 0-A is required for filling the latter in order to overcome the brake application stroke, i.e., up to the point of brake application. Moreover, a large time interval A-B is required during which the brake pressure rises to its maximum. The full braking effect is thus reached only after a long time interval 0-B, and this does not meet the speed requirements during emergency braking.